The invention disclosed herein relates generally to circuits for detecting the impending interruption of electrical power required for normal operation of electrically powered devices, and more particularly to a circuit for providing an input signal to a computing device which both serves as a reference frequency and indicates impending interruption of the power required for normal computing device operation.
It is required or desirable in the operation of a variety of electrically powered circuits, devices and equipment that, in the event of primary power failure, operation be terminated in an orderly fashion or continued until a safe condition is reached or at least critical functions be continued under power from an auxiliary source. This is true of computing equipment in which it is both necessary to shut down normal operation in an orderly manner and to provide at least limited electrical power for preserving stored programs and data, among other functions. It is also true of control systems for equipment in connection with which, if normal operation is to be terminated, at least critical functions of the equipment must be controlled to preclude unsafe or otherwise undesirable modes of operation and consequences. For example, if primary electrical power to a control system for controlling the operation of multiple unit air conditioning systems is lost, the control system should remain capable of driving dampers in the air conditioning equipment to safe positions, or if equipment operation is continued, controlling the various units in a coordinated manner to avoid grossly inefficient operation.
Data processing apparatus and control systems for many kinds of equipment include computing devices which provide signals indicative of time of day or other time references. Often, the most readily available and economical source from which such time references can be derived is the alternating voltage which serves as the primary power source for the computing or control equipment.
In general, the functions of detecting impending interruption of primary power and deriving time reference signals from a primary A.C. power source have been performed independently with separate circuitry. The applicant has, however, devised a unique simple reference frequency and low voltage detector circuit which provides a single output from which a computing device can both derive time of day or other time reference and switch to a secondary mode of operation in anticipation of interruption of suitable primary supply power.